The understanding of the phenomena occurring during immersion of LiNi0.5Mn0.3Co0.2O2 (NMC) in water is helpful to devise new strategies toward the implementation of aqueous processing of this high-capacity cathode material. Immersion of NMC powder in water leads to both structural modification of the particles surface as observed by high-resolution scanning transmission electron microscopy and the formation of lithium-based compounds over the surface (LiOH, Li2CO3) in greater amount than after long-time exposure to ambient air, as confirmed by pH titration and 7Li MAS NMR analysis. The surface compounds adversely affect the electrochemical performance and are notably responsible for the alkaline pH of the aqueous slurry, which causes corrosion of the aluminum collector during coating of the electrode. The corrosion is avoided by adding phosphoric acid to the slurry as it lowers the pH, and it also enhances the cycling stability of the water-based electrodes due to the phosphate compounds formed at the particles surface, as evidenced by X-ray photoelectron spectroscopy analysis.
Keywords: Li-ion battery; LiNiMnCoO; aqueous electrode processing; phosphoric acid; surface modification.